The broad trend that I've been able to discern is that the prevalence and availability of firearms has very little influence on the overall amount of violent crime in any particular society. There are countries where guns are highly prevalent which have low rates of violent crime, like Switzerland and Finland, and ones which have high rates of violent crime, like Brazil. In countries where gun ownership is uncommon, you've got Japan at one end and the UK at the other. According to the latest European Crime and Safety Survey, alcohol consumption plays the largest role in violent crime rates, with rates being in highest in countries where "binge drinking" (drinking a lot in a short amount of time) is most common (the UK, Belgium, the Netherlands, Denmark and Sweden).
What higher prevalence of firearms does do (all other things being equal) is make violent crime more (likely to be) lethal. For example, in Europe, if some macho idiot gets into a dispute with a nightclub bouncer, he might pull a knife. When something like that happened here in Oly a couple of years back, the punk (some gang-banger from Tacoma) went to his car, came back with a gun and opened up on the front of the club. It's simply easier to kill someone with a gun than with a blade; that's why we don't fight wars with pikes and swords any more.
Note, incidentally, that I'm not making a distinction between legally owned and illegally owned firearms. There's an oft-repeated statistic in which Northern Ireland is cited as being "low gun ownership, high homicide rate"; well, low legal gun ownership, yeah, but with three decades of the Troubles, I don't buy that there weren't an unusually (for Europe) large number of guns knocking around the province.